The ESB is to refund almost €1 million to 3,000 customers around the country following an internal review.
The refund came about following an internal review in the company where areas were reclassified from rural rates to the lower cost urban rate. Each customer will receive in the region of €300.
An ESB spokesman said the reclassification was due to "the unprecedented growth in housing".
The spokesman denied it was a case of overcharging but a reclassification of ESB levies "because of urban sprawl".
READ MORE
The review, the company's first since 1998, was carried out in preparation for deregulation of the electricity market due to take place in February 2005.